2B: Transport of materials across the plasma membrane Flashcards
- Allows some materials to pass.
->Water, oxygen, carbon dioxide
– Prevents others from passing.
->Proteins, carbohydrates
Differentially (selectively permeable)
Factors that determine how a substance may be transported across a plasma membrane:
- Size
– Polar or Nonpolar
– charge
- governs biological systems
– universe tends towards disorder (entropy)
2nd law of thermodynamics
• Only small, relatively hydrophobicmolecules are able to
diffuse across a phospholipid bilayer at significant rates.
• Molecules have to dissolve in lipid interior
PASSIVE DIFFUSION
Why is diffusion important to cells and humans?
• Cell respiration
• Alveoli of lungs
• Capillaries
• Red Blood Cells
• Medications: timerelease capsules
Cell respiration
Proton motive force
Alveoli of lungs
passive diffusion of gas exchange
Medications: time release capsules
- cover takes a while to be digested (oral)
- can be injected to the bloodstream
• Diffusion through protein channels which do not interact with hydrophobic interior
– For biological mol unable to dissolve in hydrophobic interior
– no energy needed
Facilitated diffusion
With help
Facilitated
Fast transport
open channel
The passage of materials is aided by a concentration gradient and by a transport protein
Facilitated diffusion
Two kinds of Proteins
Carrier Protein
Channel protein
- bind specific molecules, undergo conformational change to release molecule
- ex. Glucose transporters
Carrier Proteins
- form open pores for free diffusion
- found in gap junctions
Channel Proteins
Molecules will randomly move through the _______ in Channel Proteins.
pores
• Some Carrier proteins do not extend through the membrane.
• They _________ and _______ molecules through the lipid bilayer and release them on the opposite side.
bond and drag
Other carrier proteins _________ to move materials across the cell membrane
change shape
How do molecules move through the plasma membrane by facilitated diffusion?
Channel and Carrier proteins are specific:
• Channel Proteins allow ions, small solutes, and water to pass
• Carrier Proteins move glucose and amino acids
• Facilitated diffusion is rate limited, by the number of proteins channels/carriers present in the membrane.
Most cells, including erythrocytes, are exposed to extracellular glucose concentrations that are higher than those inside the cell, so facilitated diffusion results in _________
the net inward transport of glucose
• Water Channels
• Protein pores used during Osmosis
• provide corridors allowing water molecules to cross the membrane.
• Allow for fast transport
• water channel proteins make possible massive amounts of diffusion
Aquaporins
– is the diffusion of water across a differentially permeable membrane.
OSMOSIS
is the pressure that develops in a system due to osmosis.
Osmotic pressure
determined by comparing total solute concentrations
Direction of osmosis
more solute, less water
Hypertonic
less solute, more water
Hypotonic
equal solute, equal water
Isotonic
depends on balancing water uptake and loss
Cell survival
- Lysed (animal cell)
- turgid (Plant cell)
- Hypotonic solution
Freshwater
- Normal (Animal cell)
- Flaccid (Plant cell)
- Isotonic solution
- Balanced
- Shriveled (animal cell)
- plasmolyzed (plant cell)
- hypertonic solution
Saltwater
Both solutions have the same osmolarity
volume unchanged
Both solutions of water and sugar have identical osmolarity, but volume of the solution on the right is greater
Because only water is free to move
– animal cell immersed in mild salt solution
• example:
blood cells in blood plasma
• problem: none
– no net movement of water
» flows across membrane equally, in both directions
– volume of cell is stable
Isotonic
– a cell in fresh water
• example: Paramecium
• problem: gains water, swells & can burst
– water continually enters Paramecium cell
• solution: contractile vacuole
– pumps water out of cell
– ATP
plant cells
• turgid
Hypotonic
Water regulation in paramecium
Contractile vacuole
– a cell in salt water
• example: shellfish
• problem: lose water & die
• solution: take up water or pump out salt
plant cells
• plasmolysis
Hypertonic
Plasmolysis
wilt
Why is osmosis important to cells and
humans?
• Cells remove water produced by cell respiration.
• Large intestine cells transport water to bloodstream
• Kidney cells form urine
Non-lipid soluble substances diffuse through
Membrane channels
regulates the opening/closing of the channel.
membrane potential (voltage)
permit the free passage of ions and small polar molecules through the outer membranes of bacteria.
Porins
mediate the passage of ions across plasma membranes.
Ion channels
open in response to the binding of neurotransmitters or other signaling molecules.
Ligand-gated channels
open in response to changes in electric potential across the plasma membrane.
Voltage-gated channels
